A circuit unit situated in a feedback path of a negative feedback circuit generally includes circuit elements such as resistors and capacitors to ensure the stability of the negative feedback circuit. Manufacturing those circuit elements like resistors and capacitors by use of semiconductors results in the variation of parameters of these elements being relatively large. In consideration of this, a negative feedback circuit is designed to have a narrow bandwidth in order to ensure stability despite the variation of characteristics of the feedback circuit portion. A narrow bandwidth, however, gives rise to the problem of slow response time, i.e., the problem in that it takes a long time to attain convergence. Use of a narrow bandwidth in a negative feedback circuit for use in a power-supply control IC, for example, results in a large fluctuation in the output voltage occurring in response to a sudden change in the load current.
Even in the case of a relatively large variation being present in the parameters of circuit elements used in the feedback portion, the resistance values and capacitance values may be adjusted by monitoring the loop-gain characteristics, thereby providing desired stability and response speed. Specifically, the adjustment of resistance values and capacitance values is performed while measuring the phase margin and the gain margin by measuring the loop gain. These obtained margins are taken into account to determine the correction values of the resistors and capacitors. In order to measure the loop-gain characteristics, a signal source may be inserted between the output terminal of a circuit to be measured and the input terminal of a circuit in the feedback path to which the output terminal is coupled. A signal applied to the input terminal and a signal appearing at the output terminal are then measured, followed by obtaining the voltage ratio and phase difference between these signals.
Direct conversion may be utilized in order to measure gain characteristics, i.e., to measure the voltage ratio and phase difference between a signal applied to the input terminal and a signal appearing at the output terminal. Direct conversion is a signal processing method that is typically utilized in high-frequency wireless communication (e.g., in the range of a few hundred MHz to a few GHz or more). Multiplication of an input signal by a local oscillating signal having the same frequency as the input signal serves to convert the input signal directly into a baseband signal. Direct conversion uses an analog circuit to implement an AFE (i.e., analog front-end), a mixer circuit, and a low-pass filter for extracting a baseband signal. This is because converting a high-speed signal with high precision by use of an AD converter is a difficult task.
Implementing an AFE, a mixer circuit and a low-pass filter by use of analog circuits presents a difficult design challenge, and the circuit characteristics tend to vary in response to temperature changes and due to deterioration with time. Variation of the characteristics of these analog circuits between signal paths serves to deteriorate the accuracy of measurements. Further, there is another problem with direct conversion in which the phase of a local oscillating signal is supposed to be displaced by 90 degrees. Namely, it is difficult to provide a 90-degree phase displacement with high accuracy by means of analog processing with respect to various frequencies.
[Patent Document 1] Japanese Laid-open Patent Publication No. H10-164164
[Patent Document 2] Japanese National Publication of International Patent Application No. 2009-506311